[parvez2683]

Hi Friends,
I am trying to simulate flow over an airfoil NACA0012 with chord C=20mm. I have made the grid with 114400. But when I am trying to simulate it in Fluent First thing it gives me a warning:
The mesh contains very high aspect ratio hexahedral
or polyhedral cells.
The default algorithm used to compute the wall
distance required by the turbulence models might
produce wrong results in these cells.
Please inspect the wall distance by displaying the
contours of the 'Cell Wall Distance' at the
boundaries. If you observe any irregularities we
recommend the use of an alternative algorithm to
correct the wall distance.
Please select /solve/initialize/repair-wall-distance
using the text user interface to switch to the
alternative algorithm.

but since I am using laminar flow I discard the warning and started simualtion with left top and bottom farfield as velocity inlet and right farfield as outflow but it is giving me a reverse flow at the outflow face 4 at the very starting of the iterations only .
What should I do? I am really stucked with this problem and I soon have my defence I need to complete this asap. I can mail my case file if needed.

Thanks
PARVEZ

[tas38]

It sounds as if the mesh quality may be poor. The aspect ratio in some of the cells near the boundary my be too high. If you could post your mesh, that would be great.

tas38

[parvez2683]

Thanks a lot for the reply....I have attached the file links please see.

http://img10.imageshack.us/i/25241961.jpg/
http://img206.imageshack.us/i/97161438.jpg/
http://img206.imageshack.us/i/31751490.jpg/
http://img206.imageshack.us/i/81466775.jpg/

[tas38]

It looks as thought the grid was constructed with a boundary layer attached to the airfoil surface. Is this the case?

Additionally, I would focus attention on cells near the farfield. The increasing grid density near the farfield can result in high aspect ratio cells. Additionally, if the cells are non-orthogonal to the farfield boundary, they can become highly skewed.

Could you post pictures of the farfield boundary near the trailing edge of the airfoil and projected "upwards" -- near where the semi-circular edge and straight edge of the farfield boundary meet.

[Chris D]

You might be getting reversed flow because you have a face with an inlet boundary condition directly adjacent to a face with an outlet boundary condition. What happens (possibly) is that the flow from the inlet, when it enters the domain, pulls flow in through the outlet. Since you have flow "the wrong way" at the outlet face, FLUENT warns you that you have reversed flow.

[nisha]

Originally Posted by tas38

It sounds as if the mesh quality may be poor. The aspect ratio in some of the cells near the boundary my be too high. If you could post your mesh, that would be great.

tas38

Yes, so was it for my case. Edited the mesh, reduced the aspect ratio, and it worked. Thanks.